(Shameless copy of the answer at this source and possible dublicate, which extracted the information from this article.)
There are a few elements (<input>, <select>, <button>, <img>,
<object>, and <textarea>) that are considered replaced elements
whose appearance and dimensions are defined by an external resource.
(e.g. the operating system, a plugin, etc).
Replaced elements can have intrinsic dimensions—width and height
values that are defined by the element itself, rather than by its
surroundings in the document. For example, if an image element has a
width set to auto, the width of the linked image file will be used.
Intrinsic dimensions also define an intrinsic ratio that’s used to
determine the computed dimensions of the element should only one
dimension be specified. For example, if only the width is specified
for an image element—at, say, 100px—and the actual image is 200 pixels
wide and 100 pixels high, the height of the element will be scaled by
the same amount, to 50px.
Replaced elements can also have visual formatting requirements imposed
by the element, outside of the control of CSS; for example, the user
interface controls rendered for form elements.
With HTML5 you have a couple more of those like <audio> and <canvas> and some more.
Please note that - as you will see in the discussions in the comments - button is not really a replaced element defined by w3c. However it is behaving like one, which is discussed further in this article.
